The present invention relates to sense amplifier circuits for use in memory integrated circuits.
In memory integrated circuits, sense amplifiers are used to detect and determine the data content of a selected memory cell. In EEPROM (Electronically Erasable Programmable Read Only Memories) and Flash memories, the sense amplifier serves two functions. Firstly, the sense amplifier precharges the bitline to a clamped value, and secondly, it senses the current flowing into the bitline, which depends on the memory cell state. Both the reliability, in terms of endurance and retention, and the performance depend to greatly on the design of the sense amplifier.
The majority of integrated sense amplifier structures are based on a differential amplifier being used to compare the current coming from the selected memory cell to the current of a reference cell. The reference cells can be implemented in different ways, and are of different types. The reference cells are programmed one time only during the test of the memory, thus increasing the testing time. In order to ensure the good functionality of the sense amplifier, the ratio Icell/Iref must be maintained high enough to take into account the process fluctuations on the memory and the reference cells, and the impact of the memory cycling on the memory cells. Moreover, it has been shown that the speed performance and reliability of the standard differential amplifier sense amplifiers are highly reduced for supply voltage values under 2V.
Other types of sense amplifier structures are non-differential types that have nonsymmetrical circuits which detect and amplify signals which are generated by an accessed memory cell on a single amplifier input node. These types of sense amplifiers are often referred to as xe2x80x9csingle-endedxe2x80x9d. Among the single-ended sense amplifiers of the prior art is U.S. Pat. No. 4,918,341 to Galbraith et al. which discloses a single-ended sense amplifier that includes a complementary current mirror circuit that converts a single-ended input current into a single-ended output voltage. The ""341 patent also discloses a circuit for filtering high frequency noise spikes. U.S. Pat. No. 5,013,943 to Hirose discloses a single-ended sense amplifier having a precharging circuit in order to lessen the effect of changing the bitline capacitance and the variability in the current conducted by the cell. U.S. Pat. No. 5,666,310 to Yu et al. discloses a sense amplifier circuit that senses the current drawn by the memory array and changes the state of the output once a certain current has been reached.
The trend in recent years is to design memory circuits that consume less power. One way to do this is to decrease the voltages of the power supplies that provide power to the memory. As the power supply voltages used for sense amplifiers decrease, it becomes more important that the sense amplifier be able to sense very low current levels.
It is the object of the present invention to provide a single-ended sense amplifier having direct current amplification in order to sense very low currents.
It is a further object of the invention to provide a single-ended sense amplifier that can be designed using standard low voltage CMOS devices.
The above objects have been achieved by a single-ended current sense amplifier having a precharge circuit to maintain a stable voltage on a bitline, a sensing circuit coupled to the bitline for sensing an amount of current flowing into the bitline, a direct current amplification circuit coupled to the sensing circuit for amplifying the current sensed on the bitline, a current-to-voltage conversion circuit for converting the sensed current to a voltage, and a voltage amplification circuit for amplifying the voltage at the sense amplifier output. The sense amplifier also includes an overshoot filtering circuit to filter out positive glitches on the bitline.
The single-ended structure provides the advantage of eliminating the need to have a reference cell and a comparator circuit as are commonly used in the differential sense amp structures. This provides a savings in testing time and in the amount of die area used by the sense amplifier circuit. Additionally, the single-ended structure provides other advantages over the standard differential structures such as providing less sensitivity to mismatching and process variations and providing improved access time at low supply voltages.
By providing direct current amplification immediately following the current sensing, the sense amplifier of the present invention is faster and can sense very low currents compared to other single-ended sense amplifiers of the prior art.